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From: Akira Yokosawa <akiyks@gmail.com>
Subject: [Possible BUG] count_lim_atomic.c fails on POWER8
Message-ID: <d6abdc9c-9d0e-9435-05be-85a4fe9d3347@gmail.com>
Date: Sun, 21 Oct 2018 00:53:17 +0900
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To: "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc: perfbook@vger.kernel.org, Akira Yokosawa <akiyks@gmail.com>
List-ID: <perfbook.vger.kernel.org>

Hi Paul,

I just noticed occasional error of count_lim_atomic.c on POWER8 at current master.
As I've recently touched the code under Codesamples/count/, I also tested on
the tag "v2017.11.22a", and saw the same behavior.

The POWER8 virtual machine is Ubuntu 16.04.

Example output:

$ ./count_lim_atomic 6 uperf 1
!!! Count mismatch: 0 counted vs. 8 final value
n_reads: 0  n_updates: 26038000  nreaders: 0  nupdaters: 6 duration: 240
ns/read: nan  ns/update: 55.3038

$ ./count_lim_atomic 6 perf 1
!!! Count mismatch: 0 counted vs. 11 final value
n_reads: 287000  n_updates: 1702000  nreaders: 6  nupdaters: 1 duration: 240
ns/read: 5017.42  ns/update: 141.011

As you see, the final count check of zero fails even when nupdaters == 1.

I have no idea what's wrong in count_lim_atomic.c.

Can you look into this? There might be something wrong in the header file
under CodeSamples/arch-ppc64.h.

On x86_64, I've never seen the count mismatch.

         Thanks, Akira